Martina Živković, Josip Orović, Igor Poljak ANALYSIS OF STEAM TURBINES FOR FEED WATER PUMPS ON LNG SHIPS ICTS 2018 Portorož, 14. – 15. June 2018 441 ANALYSIS OF STEAM TURBINES FOR FEED WATER PUMPS ON LNG SHIPS Martina Živković, B.Sc. Antuna Barca 9D, Zadar, Croatia [email protected]Josip Orović, D.Sc. Igor Poljak, M.Sc. University of Zadar, Maritime Department Mihovila Pavlinovića 1, Zadar, Croatia [email protected], [email protected]ABSTRACT Steam propulsion plants for LNG ships have main feed water pumps that are driven with steam turbines. Those steam driven feed water pumps are one of the most important components in the plant. Their function is to dose the feed water for main steam boilers timely and exactly in order to maintain normal functioning of the steam propulsion plant. In this paper the mostly used steam driven feed water pumps are presented. Their construction details, steam temperatures, pressures, velocities, efficiencies and losses over the steam turbine stages and wide range of steam turbine operating loads are analyzed. Keywords: Steam turbine, feed water pump, working parameters, efficiency 1 INTRODUCTION This paper presents two types, Coffin and Shinko, feed water pumps that are driven with steam turbines and used to deliver the feed water to main steam boilers of LNG ships. Coffin main feed water pump is constructed as a two staged centrifugal pump [1] and Shinko main feed water pump is of three stages centrifugal type pump [2]. Both feed water pumps are driven by the short and light weight Curtis wheel steam turbine [1, 2]. Feed water pumps take water suction from the highest point in the engine room, from the deaerator which acts as a header tank, and deliver the feed water with high pressure, over 60 bar, into the main steam boilers. Before it enters the main steam boilers the feed water temperature is usually raised passing through the high pressure feed water heater and economizer in order to reduce the fuel oil consumption in the boilers and to increase the overall efficiency of the steam plant. The main steam boilers level control and thus the capacity control of main feed water pump is often a three term control. This control measures water level, steam and water flow and controls the output to the boiler feed water control valves. The steam flow to the steam turbine and the running speed of feed water pump is controlled by the differential discharge pressure control. This control compares the feed water discharge pressure with boiler steam drum pressure and adjusts the feed water pump speed by regulating the steam flow across the steam turbine in order to maintain the constant difference across the feed water regulator [3]. Coffin feed water pump, shown in Figure 1, consists of four sections: turbine assembly, bearing housing assembly, governor gear and oil pump assembly and pump assembly. At the one end, the driving steam enters the single stage, impulse, two bucket row, velocity compounded, axial flow turbine and expands through the turbine wheel and then exhausts in an axial direction directly above the turbine end cover. A single shaft, used for driving the two stage centrifugal pump at the other end, passes through the turbine housing, bearing housing, and pump housing. This shaft is supported by two roller bearings located in the bearing casing and the thrust bearing located on the outboard end of the pump housing. The bearing casing additionally serves as an oil reservoir and for providing the power to the horizontal governor drive shaft through a worm and a worm gear. Feed water pump control is accomplished by the pump discharge pressure via constant pressure regulator, through centrifugal speed governor when rated speed has been exceeded and through excess exhaust back pressure trip and low lube oil pressure trip protection [1]. Outer design of one Shinko pump is shown in Figure 2. It consists of feed water pump assembly and steam turbine assembly interconnected by the gear coupling. Feed water pump is also multi stage centrifugal pump (in this case three stages) where the first stage impeller is of the double suction type and the rest are of the single suction type. It has horizontally split casing in order to have easier maintenance. Rotor is supported by plain forced lubricated bearings and tilting pad type thrust bearing. Feed water pump is directly connected to the driving steam turbine but in this case through a forced lubricated gear coupling. Steam turbine is of the horizontal single stage speed compound impulse type. The turbine casing is also horizontally split into two parts. Bearing housings are at the both ends of the turbine shaft. The governor end bearing housing is provided with radial bearing and thrust bearing. The bearing housing near the feed water pump has radial bearing only. For the control of the steam turbine
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ANALYSIS OF STEAM TURBINES FOR FEED WATER PUMPS …turbine revolutions, ψ is friction loss in rotor blades, m is steam mass flow in kg/h, P is steam turbine power and η T is steam
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Martina Živković, Josip Orović, Igor Poljak
ANALYSIS OF STEAM TURBINES FOR FEED WATER PUMPS ON LNG SHIPS
ICTS 2018
Portorož,
14. – 15. June 2018
441
ANALYSIS OF STEAM TURBINES FOR FEED WATER PUMPS ON LNG